In recent years, the use of advanced composite materials to fabricate structures has experienced tremendous growth in the aerospace, automotive, and many other commercial industries. While composite materials offer significant improvements in performance, they require strict quality control procedures in both the manufacturing processes and after the materials are in service in finished products. Specifically, non-destructive evaluation (NDE) methods must be developed assess the structural integrity of composite materials in an efficient and cost-effective manner. This assessment detects inclusions, delaminations and porosities within the internal structure. Conventional NDE methods are slow, labor-intensive, and costly. As a result, testing procedures have historically adversely affected the manufacturing costs associated with composite structures.
Various methods and apparatuses have been proposed to assess the structural integrity of composite structures. One solution uses an ultrasonic source to generate ultrasonic surface displacements which are then measured and analyzed. Often, the external source of ultrasound is a transducer mechanically coupled to the target to be tested or a pulsed generation laser beam directed at the target. Laser light from a separate detection laser is scattered by ultrasonic surface displacements at the target. Collection optics then collect the scattered laser energy. The collection optics are coupled to an interferometer or other optical processing device to produce an output or data signal. Data about the structural integrity of the composite structure is then obtained through analysis of the scattered laser energy.
Laser ultrasound has been shown to be very effective for the inspection of composite materials during the manufacturing process. These inspections have been complicated by parts having non-uniform thickness or non-parallel surfaces. To date, laser ultrasound has been limited to parts having parallel surfaces.
During ultrasonic inspection, echoes reflected by the interior surfaces of a sample allow measuring the sample thickness. Additionally, this echo provides an indication of how to position the detection time gate. When target materials have non-parallel surfaces, the measurement of this echo becomes complicated as the ultrasonic wave does not return to the point of origin.
Unfortunately, laser ultrasound is limited to parts having parallel surfaces. The benefits associated with ultrasonic inspection cannot be applied using current techniques to parts having non-parallel surfaces. These parts constitute a significant fraction of the total composite material production, especially in the aircraft industry.